1. Field of the Invention
The present invention relates to torque control of a robot.
2. Description of Background Art
Often, a robot is commanded to follow a given trajectory in order to accomplish a particular task. Individual joint position commands are calculated by applying inverse kinematics for an end-effector's position level control in Cartesian space. Motion can also be accomplished by directly commanding individual joint positions. A typical position controller by Proportional-Integral-Derivative (PID) control is implemented for each joint level controller and the joint position command is achieved by high gain control. This is a popular control system used with industrial robots. It is used because accuracy and quick responses are required to accomplish desired tasks and these were achieved by traditional position control.
Many humanoid robots are also controlled by position control. Traditional position controllers have been successfully applied for humanoid robot control, particularly for balancing and walking control. This is because accuracy and fast responses for Zero Moment Point (ZMP) control and manipulation are of a high priority for humanoid robots. Human-like whole-body motions and simple manipulation tasks are also often realized by using traditional position control.
More advanced robots are desired to work not only in industry but also in the everyday environment of people. These robots can be used to assist people and to communicate with people in their homes, offices, public spaces, hospitals, disaster areas etc. One of the critical problems for controlling a robot in people's daily environments is (i) contact between the robot and the external environment and (ii) contact with people. In case of the contact with the environment, unpredictable contact will happen because the work space of the robot is very narrow and complicated. In case of the contact with people, human motion itself is unpredictable. However, many advanced robots are not designed for contact. They are designed for the presumption that the external environment is fixed or that contact points are limited (e.g., both hands and feet). As a result, development of compliant motion for safe contact with the environment and with people is an important issue for advanced robot control.
Whole-body contact and control are often used for many small robots. Because a small robot has a lower weight, the contact force between the robot and the external environment is negligible. Also, in many cases, small robots are controlled by position command which is generated by kinematics without accounting for any contact force. As for human-sized robots, whole-body motion has been used with some robots. These robots can stand up from a prone position or crawl in a narrow space. However, the motion is pre-designed and the robot is just following the motion commanded by position control. Compliant control is applied to motion in contact; however, the motion while in contact with the environment is limited to a fixed or stable environment. To realize more advanced whole-body motion in contact with the environment or with a human, the robot's posture should be compliant with external forces so that the robot can accomplish its multiple tasks smoothly by sensing the environment through physical contact as a human would.
One solution for this problem is use torque control for robots. Often, to realize torque control, the control system hardware is designed for torque control. The motor is controlled by a torque control unit by which measured torque feedback data is compensated and a desired torque command input is achieved. However, a torque controlled system uses significantly different hardware from a position controlled system, which is commonly found in robots. As a result, it can be difficult to use torque control with a position controlled system. What is needed is a method for realizing compliant open-loop torque control on a traditional position-controlled system with position-control hardware.